1. Field of the Invention
This invention relates to an image forming device to form an image on an image recording member. More particularly it is concerned with an image forming device, such as, for example, an electrostatic recording device based on the electrophotographic process, having a function of stabilizing the image formed on the image recording member.
2. Description of Prior Arts
An electrostatic recording device to form an electrostatic latent image on a recording member such as a photosensitive member, an insulative member, etc. has already been known. Such known electrostatic recording device will be described hereinbelow by taking a reproduction device based on the electrophotographic process, as an example.
FIG. 2 of the accompanying drawing illustrates how surface potentials on a photosensitive drum corresponding to a bright portion (a portion with more light reflection) and a dark portion (a portion with less light reflection) vary at each processing position in the known reproduction device during the reproduction process. Of these potential variations, required as the final electrostatic latent image is the surface potential at a point .circle.C in the graphical representation, wherein the surface potentials .circle.a and .circle.b at the dark portion and the bright portion, respectively, vary as shown by .circle.a' and .circle.b' in FIG. 3 when an ambient temperature of the photosensitive drum increases, and they also vary with respect to aging of the photosensitive drum with passage of time as shown by .circle.a' and .circle.b' in FIG. 4 with the consequent inability to obtain an image contrast between the dark portion and the bright portion.
Compensation for variations in such surface potentials is disclosed in British laid-open patent application No. 2039101 which was filed by the same assignee-to-be as the present application.
However, when the photosensitive member in the reproduction device is replaced by other photosensitive member of totally different characteristics, the control programs which have been prepared for the former photosensitive member become useless at all owing to such difference in the characteristics, which necessitates re-arrangement of the control program.
In addition, when an image of a variable magnification is to be obtained by changing a process speed, for example, there occurs such a phenomenon that the image density to the same surface potential becomes low with a high process speed, and it becomes high with a low process speed, provided that developing capability of a developer does not change, or remains constant.
Further, when humidity in the surrounding atmosphere increases, there occur variations in the charge characteristic coefficients .alpha..sub.1, .alpha..sub.2, .beta..sub.1, .beta..sub.2 (to be described later) of the photosensitive member with the consequence that the control effect cannot be displayed so remarkably as expected by the control method as disclosed in the abovementioned British specification No. 2039101.